1. Field of the Invention
The present invention relates to an insulation testing device for a gas insulated apparatus, and particularly relates to an insulation testing device for testing an initial state of defective insulation by effectively detecting electrically-conductive (hereinafter simply referred to as "conductive") foreign particles attached to the surfaces of insulators of a gas insulated apparatus, which become harmful in the operating condition.
2. Description of the Prior Art
Heretofore, in general, an AC voltage of a commercial frequency has been used as a test voltage when an insulation test of a gas insulated apparatus is carried out at a place where the gas insulated apparatus is installed, as described in "Electrical Cooperative Research", Vol. 39, No. 6, February in 1984, page 133, issued by The Society of Electrical Cooperative Research. According to a practice in Japan, a relatively low voltage equivalent to 110% of an operating voltage of a gas insulated apparatus has been employed as a test voltage used in the test of the apparatus in the location where the apparatus is installed, because it has been supposed that the final test at the factory has been sufficiently carried out for the gas insulated apparatus.
On the other hand, the insulation property of a gas insulated apparatus is often remarkably lowered unlike that of an conventional air insulated apparatus when contaminated with fine conductive foreign particles. particularly under the condition that an AC voltage of a commercial frequency is applied, conductive foreign particles in an applied electric field are cause to float by electrostatic force so as to stray within the gas insulated apparatus, because the duration of voltage application is too long. The conductive foreign particles which are present in a relatively low intensity electric field in the initial state of an insulation test, may be moved and attached to a part of the insulated apparatus so as to be harmful to the insulation. Generally, lowering of insulation little occurs when the conductive foreign particles stray in a gas space, but great lowering of insulation occurs when the foreign particles are attached to the insulator surface. Accordingly, to maintain the insulation reliability of the gas insulated apparatus for a long time, the insulation test voltage at the location of installation of the gas insulated apparatus must be made as high as possible. From this point of view, outside Japan, an AC voltage of a commercial frequency equivalent to 75 to 80% of an insulation test voltage at the factory has been employed as the test voltage as described in the aforementioned paper. The voltage shows a value 1.7 times to 2 times as much as a normal operating voltage.
Recently, a tendency to use a switching impulse voltage for the on-site insulation test is seen, as discussed in the paper "High Voltage Testing of Metal-enclosed, Gas-insulated Substations On-site With Oscillating Switching Impulse Voltage", by K. Feser, HAEFELY El-44, 1979/7.155
However, the aforementioned technique has two problems as follows.
One of the problems relates to the value of the on-site test voltage. Application of a voltage higher than an AC voltage of a commercial frequency is most effective in view of reliability, but the maximum value of an AC voltage of a commercial frequency in the operating state is not larger than about 130% of the rated operation voltage of the apparatus. Application of a voltage higher than 130% of the rated operation voltage induces a possibility that conductive foreign particles harmless in general may be moved to a part of the insulated apparatus so as to become harmful to the insulation.
The other problem relates to the applied voltage waveform in the actual operating state.
It is supposed that the excessive voltage over 130% of the rated operation voltage is different in waveform from a commercial frequency voltage but has a waveform of a short-duration switching impulsive voltage. Further, it is necessary to determine the maximum value of the applied voltage taking even a limit voltage of a lightning arrester or the like into consideration.